Multicystic dysplastic kidney is a renal dysplasia resulting from a complete obstruction at either the renal pelvis or proximal ureter prior to the 10th week of gestation. It is the second most frequent abdominal mass found in a fetus after hydronephrosis. The kidney becomes non functioning, and it is composed of non communicating cysts of variable dimensions. The renal parenchyma is not individualized. Cysts vary from 1 or 2 cm up to 10 or 15 cm. They tend towards involution in the postnatal period, although some intrauterine involution may happen. This dysplasia is unilateral in 95 percent of cases; and it is incompatible with life, if bilateral. There is an association with contra lateral anomalies in about 40 percent of cases, being stenosis of pyeloureteral junction and vesicoureteral reflux the most frequent anomalies. A dysplastic multicystic kidney may get segmented before a complete pyeloureteral duplication. If unilateral, the volume of amniotic fluid remains normal. The main differential diagnosis is that made with pyeloureteral junction. The difference is the localization of the biggest cyst, which is, in cases of junction stenosis, centrally localized with the other communicating cysts close by.